Resistor making method



NOV. 4, 1941. ow ETAL 2,261,916

RESISTOR MAKING METHOD Filed July 15, 1937 3 Sheets-Sheet l INVENTORS NW5 paw Nov. 4, 1941. MEGOW AL 2,261,916

RESISTOR MAKING METHOD Filed July 15, 1937 3 Sheets-Sheet 2 lNVENTO our BY Amwuf mm 0144 QMWZV, ATTORNEY Nov. 4, 1941. G E, ME OW ETAL 2,261,916

RES I STOR MAKING METHOD 4 Filed July 15, 1937 3 Sheets-Sheet 3 g & INVENTORS L017 I aw- BY H ATTORNEY.

Patented Nov. 4, 19 41 UNITED STATES PATENT ."OFFICE RESISTOR. MAKING METHOD George E. Megow, South Milwaukee, and Homer "G. Thomson, Milwaukee, Wis., assignors to Allen-Bradley, Company, Milwaukee, Wis., a

corporation of Wisconsin Application July 15, 1937, Serial No. 153,746

2 Claims.

The invention relates to a resistor making method for producing a molded electrical resistor of the type in which a plastic resistant body is provided with metallic terminals or electrodes.

The terminal employed in that type of resistor is ordinarily provided with an attenuated ductile shank protruding from the resistant body to connect the resistor in circuit and a head formed upon the shank and embeddedin the resistant body to attach the terminal thereto as a unitary part thereof.

According to the way such a resistor has heretofore been made,-difliculty has been experienced in locating the head in position in the resistant body to obtain a resistance of a definite order between the terminals and in compressing the resistant body about the head in such a way as to obviate impairing intimate electrical and mechanical contact between the head and the resistant body upon bending the shank or otherwise applying stress between the head and the resistant body or upon expansion and contraction oi the resistant body about the head under variations in temperature.

The present invention has for its object to provide a resistor making method by which each terminal will have its head located in a fixed ance of a definite order between the terminals,

and the resistant body will be compressed entirely around the head and over a retaining ledge thereon to retain the head in permanent intimate electrical and mechanical contact with th resistant body.

Another object is to provide a resistor making method which will facilitate and expedite embedding the head of each terminal in the resistant body in such a way as to obtain resistance of a definite order between the terminals'and to retain permanent intimate electrical and mechanical contact between the head and the resistant body.

Another object to provide a resistor making method by which will be produced a resistor in which intimate electrical and mechanical contact between the head of the terminal and the resistant body will be maintained and in which the resistance between the terminals will thereby remain constant under the same electrical conditions.

Another object is to provide a resistor making method by which. a resistor possessing a resistance of a definite order between its terminals tions in conditions may be readily reproduced in manufacturing the resistor.

Another object is to provide a resistor making method by which an efiicient and durable electrical resistor may be readily and economically manufactured. I

According to the resistor making method constituting the present invention, the terminal is attached to the resistant body and its head located in a fixed position therein by applying pressure direct to its head from an external source to embed it in the resistant body and compressing the resistant body by applying pressure direct thereto from an external source entirely around the head to force the resistant body over a retaining ledge on the head and thereby clamp the head to the resistant body inpermanent :rvifimate electrical and mechanical contact there- The resistor making method constituting the present invention produces a resistor in which each terminal has its head located in the resistant body and clamped thereto in such a way as to obtain a resistance of a definite order between the terminals and maintain permanent intimate electrical and mechanical contact between the head and the resistant body under variations in electrical and temperature conditions and in which the resistant body about the trical resistor by the method constituting the present invention, and the following description thereof and the way the resistor thereby will serve to explain further the way to perform and practice the invention claimed therein.

The accompanying drawings illustrate in the following views the apparatus and the resistor produced thereby:

Fig. 1 is a perspective view of a suitable partially condensed blank ready for treatment by the method of this invention;

Fig. 2 is a cross-section of a suitable recessed heated die in which has been inserted a blank;

Fig. 3 is a cross-section of the die of Fig. 2 showing shaping plungers after performing a preliminary shaping step;

. and maintaining such resistance under varia- Fig. 4 is a cross-section showing centrally is made bored plungers carrying headed wire electrode inserts being inserted into the heated recessed die;

Fig. 5 is a cross-section showing centrally bored plungers carrying headed wire electrode inserts in the act of applying full molding and embedding pressure, travel being limited only by the volume of the resistor material;

Fig 6 shows the finished resistor and plungers after being ejected from the heated recessed die;

Fig. 7 shows a finished resistor after stripping the plungers from the electrode inserts;

Fig. 8 shows in section another form of heated recessed die and plunger carrying both electrode inserts adapted for side molding;

Fig. 9 shows a finished resistor formed in the die of Fig. 8;

Fig. 10 shows an enlarged partial cross-section of a resistor of this invention;

Fig. 11 shows an enlarged partial cross-section of a resistor of this invention showing bending of an electrode lead; and

Fig. 12 shows an enlarged partial cross-section of another form of the resistor of this invention.

In one instance the method of this invention may be practiced as follows: Carbon black, suitably prepared for molded electrical resistor purposes, is incorporated in the usual proportions with relatively uncondensed resin, such as phenol-formaldehyde, and a filler, such as ground quartz. The mixture is blended upon kneading rolls, or other suitable mixing device. sufficient heat being employed to partially condense the resin bonding agent and form a weak solid. The material thus prepared is then ground to a powder and fed to a pill machine, or other similar device well known in the art and not shown or described herein, which subdivides the ground mixture into uniform portions of desired mass and compresses the subdivided masses into relatively weak but solid blanks of uniform dimension and mass.

One of such blanks is illustrated in Fig. 1,

which shows an elongated hexagonal prism of partially condensed mixture of resin bonding agent and finely divided conductor and filler, the same being designated by the numeral I. The particular shape of the blank is not important, almost any definite shape being suitable. The

blank may even consist of a carefully measured I quantity of the ground material, the forming of the same into a semi-solid blank not being essential.

Blank I, prepared as above and forming the resistant body in the ultimate resistor, is then inserted, as shown in Fig. 2, in' the cylindrical hole 2 in the recessed die 3. .Recessed die 3 is shown surrounded by a heating coil 4 through which suflicient electric current is caused to flow to maintain the die block 3 at a temperature of 300 to 500 degrees F., which limits are appropriate to the particular binder mentioned above, although other binders obviously may require other temperatures. While using some binders the temperature may lie between 200 and 600 degrees F., depending upon the temperature at which the binder employed will mold properly.

The die block 3 is furnished with a pair of bearing supports 5, 5, upon which are mounted rocker arms 6, 6, joined by toggle links I, I, one each making pivotal connection with the upper ends of the rocker arm 6, 6, at the points 8, 8. The toggle links I, I, are brought together at a pivoted connection 9 upon which is mounted the spring-backed presser foot "I upon which force may be applied.

The blank I having been inserted in the die block 3, as above described, a pair of preforming plungers II, II, as shown in Fig. 3, are inserted one in each end of the hole 2 and pressure applied upon the presser foot I0. The pressure thus applied causes the toggle linkage to act upon the rocker arms 6, 6, and to apply a force to each of the preforming plungers II, II, causing them to approach one another. The plungers II, II approach until the volume remaining between them is equal to that of the blank I, causing blank I to assume the shape shown in Fig. 3.

During this step'the substance of blank I is rapidly acquiring a temperature approximating that of die' block 3 and a resumption of the condensation of the resin binder is taking place. Before the course run by the condensation reaction becomes substantial in amount, preforming plungers II, II are withdrawn in preparation for succeeding steps.

Upon withdrawal of the preforming plungers II, II, a pair of centrally bored electrode-carrying plungers I2, I2 are inserted in the ends of the hole 2, as shown in Fig. 4. The electrodecarrying plungers I2, I2 carry in their central bores headed wire'terminal electrodes I3, I3, having relatively nondeformable heads 21 and easily flexed wire shanks 26. Both the electrodes I3, I3 and the carrier plungers I2, I2 are shown more in detail in Fig. 6.

Pressure is next applied upon the presser foot I0, forcing the carrier plungers I2, I2 toward one another to the position illustrated in Fig. 5. a

When this is done the only agency limiting the travel of the plungers I 2, I2 is the volume occupied by the body portion I of the resistor and, as a consequence, the pressure applied thereto may be maintained at any full value desired from moderate pressure up to 2000 lbs. per sq. in., or heavy pressure up to and beyond 50,000 lbs. per sq. in., thereby performing molding at controlled pressure rather than at constant volume. By the time this state of the process is reached the body I has acquired a temperature approximating that of the die block 3 and the condensation reaction moves toward completion.

The preforming step shown in Fig. 3 may be omitted. The forming of the resistor blank into the finished resistance unit with the electrodes.

embedded therein may be done entirely by the electrode-carrying plungers I2, as shown in Fig. 4.

Pressure may be intermittently applied upon the presser foot III to facilitate various methods of machine or automatic molding. In such large production methods the resistor blank may be inserted into a die and the die moved on to succeeding stations and at each such station pressure may be applied by a presser foot located at that station. The intermittent application of pressure, furthermore, facilitates escape of volatile products resulting from the curing of the resistor blank.

The method of the invention may obviously be employed to produce a wide variety of sizes, values, and forms of resistors from a wide variety of ingredients. As an instance illustrative of the invention applied in a specific case, from which conditions for other forms may be gauged, a 100,000 ohm resistance unit may be made as follows: Lump resin (phenol-formaldehydetrade name and grade Durez) 25%; calcined carbon black 12%; ground quartz 62%; and lubricant 1%; which have been mixed at 225 degrees F. for a period of one-half hour, are then ground, reacted at 250 degrees F., and pressed into-blanks having a weight of 0.165 gm. each. A recessed die having a. cylindrical opening 0.140 in diameter and heated to a temperature of 425 degrees F. is used for the molding operation. Under these conditions the blank, after having been struck to preliminary shape, is subjected, during the steps of embedding the electrode inserts, to a heavy pressure of 20,000 lbs. per sq. in., for approximately one-half second, the pressure then being relieved for three seconds, followed by repeated one-half second applications of a pressure of 20,000 lbs. per sq. in. The pressure is thus applied and relieved alternately three times and after being relieved the last time the resistor is allowed to remain in the mold for thirty seconds, at the end of which treatment the molded resistor, with embedded electrodes and attached plungers, is ejected from the recessed die. The plungers are then stripped from the electrodes, yielding the finished resistor.

The finished resistor may then be subjected to a heat treatment to further stabilize the .electrical properties. For this purpose the resistor manufactured as described above may be heated to a temperature ranging between 300 and 350 degrees F. for a period of about fifteen to twenty hours. This latter treatment, although advantageous, may be omitted andthe resistor used for all but exacting requirements.

The electrode inserts, as shown clearly in Fig. 6, are formed from wire-preferably copper wire, solder-coated-of a size permitting easy flexing, with a relatively nondeformable head upset upon the end. For mechanical reasons the head may be formed, as shown, in frustro-conical form or other configuration rendering the same rigid and relatively nondeformable. If desired, the heads may be preliminarily coated with graphite or other finely divided material to reduce thecontact resistance at the interface. In practicing our invention any suitable bonding agent moldable under heat and pressure may be employed, including thermoplastic or heatcondensible bonding agents with or without modifiers. Further, the conducting material dispersed in the bonding agent may be any suitable, finely-divided or dispersed conductor.

In Fig. 8 is shown a form of recessed die adapted for molding from the side. It is composed of a block l4 surrounded by a heating coil l5 through which electric current may be passed to raise the temperature of the die block to the desired point. The block I4 is recessed to create a rectangular, upwardly-facing slot I6 resting upon the bottom of which is the fiat ejector shoe IT. The shoe I! may be raised by the rod l6 to eject the contents from the slot I6. A closely fitting rectangular plunger die [9 is arranged to enter the slot l6 and move vertically therein. On either side of the plunger die l9 are holes 20, 20, drilled as shown to receive headed wire electrode inserts. Extending above the plunger die I9 is the guiding head 2| upon which is guided the. spring-backed presser foot 22. The presser foot acting through spring 23 applies a downward pressure, as indicated by the arrow, upon the plunger die l9. v

In performing the method of this invention in the apparatus shown in Fig. 8, a suitable blank, laid in the recess I6, is struck, if desired, by a ing plunger is then withdrawn and the electrodecarrying plunger I9 is forced into the recess I6 to the position shown in Fig. 8. Full controlled molding pressure is then applied, causing the electrodes 24, 24 to become embedded, as shown. During the application of pressure the bonding agent, under the influence of heat and controlled pressure, fixes the final form of the resistor and presses uniformly against the electrodes. bringing about reliable contact resistance and good mechanial retention.

Upon completion of molding, the finished resistor is ejected by the shoe 11. The appearance of the finished resistor is illustrated in Fig. 9, wherein the numeral 25 represents the body portion of the resistor and 24, 2| the embedded wire electrodes.

The resistor formed in accordance with this invention and as illustrated in the drawings may be formed with the embedded electrodes entering from side or end and, in such cases as occasion may require, the resistor may be formed with but one embedded electrode. The resistor as shown in greater detail in Figs. 10, 11, and 12, however, is characterized by a main body portion, preferably'of controlled pressure-molded resistor material with head portions only of electrodes embedded therein.

Distinct advantages in the way of mechanical strength arise out of the novel relationship 'of the headed electrodes to the body portion of the resistor. These advantages arise in any case, but are particularly marked when the body portion has the high strength characteristic of a resistor molded under controlled pressure.

' Referring to Fig. 10, the body portion I is shown greatly enlarged, with portions of the body shown in section. Electrode leads 13 are shown comprising relatively fiexible shank portions 26, with head portions 21 joined thereto 'at the shoulders 26. The shoulder 28 on each head forms an exposed abutment upon which to apply pressure direct to the head from an external source. The

. heads 21, as shown, are slightly tapered to aid in their retention, and the inclined sides thereof form retaining ledges behind the abutment 28. The particular shape, however, is a matter of secondary importance, since any suitable configuration is appropriate as long as the same is sufficiently large and massive with respect to the diameter of the shank portion 26 as to render the same relatively nondeformable. It is also essential that the electrode head will anchor properly in the resistor and will be properly supported by the electrode-carrying plunger during the molding and embedding process. The electrodes 20 in Fig. 8 show one variation in form which is suitable.

In assembling a resistor in a circuit the shank portions 26 of the leads are frequently bent in some such manner. as illustrated at the right of Fig. 11. It is tobe noted that the bent portion of lead forms at a comparatively large radius and rupture or breakage of lead, as in a small radius, is precluded. The formation of the large radius may be augmented by cold working and/or enlargement of the neck portion of lead.

The provision or the shoulder 28, or its equivalent, facilitates the embedding of the electrode in the body portion l, according to the method of this invention, in the exact position desired.

In order to obtain the advantages of this invention, it is also possible to embed a portion of the head portion 21 within the resistor body I,

not is assured.

as shown more clearly in Fig. 12. In Fig. 12 a body portion I is shown, parts being broken to show a section. Embedded in the body portion l are leads l3 having head portions 21' joined by shoulders 28 to shank portions 26'. As shown clearly, only a part of the head portion 21' is embedded in the body portion I. Embedding the electrodes in this manner is easily accomplished by counterboring the ends of the bored plunger dies, which cause the embedding of the electrodes, or in any other manner desired.

The enlarged electrode head also has the effect of enhancing the heat-dissipating capacity. of the resistor. The high conductivity of the electrode material rapidly carries heat from the interior of the resistor to the exterior, having a direct influence upon the heat-dissipating capacity and, therefore, increases the wattage rating of the resistor. v

No excess material need be included in the blank, as all of the material is formed into the resistor body. The plungers employed in this invention fit the recessed die as closely as possible and thefinished resistor is practically free of molding flash. In contrast to this, in the old,

constant volume molding, it was essential that a slight excess of material be included in the blank so that the mold would be certain to-be filled. Such a mold, further, cannot be closed completely for the reason that the excess material of the blank must be permitted to escape into the flash of the mold. The resistors of this invention, illustrated in Figs. '7, i0, 11, and 12, have a straight,

' cylindrical body portion of nonvarying cross-section extending the entire distance between the electrodes. The only molding flash borne by the resistor is confined to two peripheral, circular flashes disposed one at each end and disposed exteriorly to the portion of the resistor which carries the current and determines the resistance.

The resistance unit of our invention may be made of comparatively small size in that accuracy in are obtained by applying pressure direct to the head from an external source to embed'it in a fixed position in the resistant body and leaving the resistant body around the head exposed and applying pressure direct thereto from an external source to compress it upon the head. v

It is desired that the protection of Letters Patent afforded hereby extend to the full extent of the inventive concept hereof as represented by the scope of the claims appended hereto without being limited by statements herein which are made in connection with the setting forth of specific instances of practice or use.

What we claim as our invention and wish to secure by Letters Patent is: t

1. A resistor making method for producing a resistor provided with terminals forming a unitary part thereof, consisting in placing together in endwise alinement with each other a plastic resistant body preformed from a plastic substance containing a comminuted conductive substance and a metallic terminal provided with a contact head abutting said resistant body and having a retaining ledge and an attenuated shank of the position of electrodes and leads with reference to body portion of resistor is attained. Also, as each part of the resistance unit is identically formed and processed, a high uniformity of prod- The-resistor produced by the apparatus and method described herein is provided with tenninals which are attached thereto in such a way that each terminal has its head embedded in the resistant body in a fixed position in relation to the other terminal and clamped to the resistant body in this fixed position by the resistant body being compressed around the head and over a retaining ledge thereon.

The terminals thus have the heads thereof attached to the resistant body in permanent intimate electrical and mechanical contact therewith to maintain a resistance of a definite order between the terminals.

The resistant body is left free to expand and contract around the head independent of the position of the head therein so as to obviate displacing the head and thereby impairing the intimate electrical and mechanical contact between the head and the resistant body.

The foregoing characteristics possessed by the resistor produced by the method described hereextending rearward from said head, applying pressure from an external source direct to said contact head in an endwise direction to force the front portion of said head into intimate electrical and mechanical contact with said body and to surround the lateral portions of said head entirely by the end of said body, thereafter applying pressure direct to the end of said body in an endwise direction entirely around said head to force the end of said body over said retaining ledge and into intimate electrical and mechanical contact with the lateral portions of said head to clamp said head to said body, and hardening said body to hold said head in permanent intimate electrical and mechanical contact therewith.

2. A resistor making method for producing a resistor provided with terminals forming a unitary part thereof, consisting in placing together in endwise alinement with each other a plastic resistant body preformed from a plastic subpressure and to surround the lateral portions of said head'entirely by the end of said body, thereafter applying pressure direct to the end of said body in an endwise direction entirely around said head to force the end of said body over said retaining ledge and into intimate electrical and mechanical contact with the lateral portions of said head to clamp said head to said body and leave said shank free from said body at its juncture with said head, and hardening said body to hold said head in permanent intimate electrical and mechanical contact therewith.

GEORGE E. MEGOW.

HOMER G. THOMSON. 

